The fact that ozone can be manufactured has been known for a long time (c.f. Werner Von Siemens “Ozonizer” circa 1857-1858). The Siemens apparatus operated to produce an electrical discharge as a “corona” or silent discharge. Ozone has been commercially generated in air or oxygen by means of such a corona discharge at a very high voltage ever since then. However, there are concerns that this technology is somewhat inefficient and rather costly in today's market. It has been reported in US2003/209447 A1 that yields in the corona discharge process generally are in the vicinity of 2% ozone, i.e. the exit gas may be about 2% O3 by weight. Obtaining this concentration requires that the input gas is cold and dry and that the system is cooled, typically with large quantities of cold water. The concentration of O3 in the exit gas can be increased by using cold, dry, pure oxygen as the input gas and O3 concentrations in the exit gas of up to 12% have been reported. However, this requires the supply or production of the pure oxygen at extra cost and requiring additional energy. Such O3 concentrations, while quite poor in an absolute sense, are still sufficiently high to furnish usable quantities of O3 for many commercial purposes, which explains why the corona discharge methodology has persisted commercially. Using pure oxygen also overcomes another disadvantage of the corona discharge process, which is that it oxidises any nitrogen in the input gas to produce harmful nitrogen oxides (NOx). Thus, other than the aforementioned electric discharge process, there is no other commercially exploited process for producing large quantities of O3.
Ozone may also be produced by an electrolytic process, wherein an electric current (normally D.C.) is applied across electrodes immersed in an electrolyte. The electrolyte includes water, which in the process dissociates into its respective elemental species, O2 and H2. Under the proper conditions, the oxygen is also evolved as the O3 species. The evolution of O3 may be represented as:3H2O→O3+3H2; ΔH0(298 K)=868.6 kJ mol−1 
Therefore, from a commercial standpoint, the known electrolytic process may be viewed as thermodynamically unfavourable in comparison with the established corona discharge process.
Nevertheless, generation of ozone in aqueous media by direct electrolysis is still a technically attractive goal, particularly if dissolved ozone is needed (ozone is poorly soluble in water).
There have been some proposals to develop the evolution of ozone by electrolysis of various electrolytes utilizing very low electrolyte temperatures, but discouragement to commercial exploitation of these proposals is the need to maintain the necessary low temperatures, requiring costly cooling equipment as well as the attendant additional energy cost of operation.
There remain many commercial drivers towards successful exploitation of electrolysis-based ozone production. The efficient, electrochemical generation of ozone is attractive due to the many uses of such a ‘clean’ and powerful oxidant. Interest in this field increased significantly in the 1980' s, with increased focus on water disinfection and detoxification.
The following patent publications are generally indicative of background art describing prior proposals to utilise electrolytic processes for ozone generation or which produce ozone as a by-product.
U.S. Pat. No. 3,256,164 (Jun. 14, 1966), Donohue, John A., et al. Electrolytic Production of Ozone. In this method of producing ozone, an electric current is passed through a liquid electrolyte, hydrogen fluoride containing not more than 10 weight percent of water, to produce a mixture of gases which contains ozone in large amounts. The procedure is carried out at a temperature of not more than 50° C. but preferably between −20° C. and +20° C. At least one weight percent of water must be present so that the electric current may pass through the liquid hydrogen fluoride and so that oxygen is present which may be converted to ozone. However, hydrofluoric acid is avoided in commercial systems due to the dangers it presents to health.
U.S. Pat. No. 4,316,782 (Feb. 23, 1982) Foller, Peter C., et al. Electrolytic Process for the Production of Ozone, discusses an electrolytic cell for production of ozone with current efficiencies of up to 52%. The cell uses a solution of highly electronegative anions, preferably hexafluoro-anions of phosphorus, arsenic, or silicon. The anode is made of either platinum or lead dioxide, and the cathode is made of platinum, nickel, or carbon. When a direct current is applied using the electrodes, ozone and oxygen are produced at the anode, and hydrogen gas is produced at the cathode.
U.S. Pat. No. 4,375,395 (Mar. 1, 1983) Foller, Peter C., et al. Process for Producing Ozone, describes an electrolytic cell for production of ozone at high current efficiencies which uses glassy carbon electrodes in an electrolytic solution containing highly electronegative BF4− or BF6− anions. These electrodes are resistant to corrosion by the electrolytic fluorine-anion-containing solution used to produce ozone at the anode. A disadvantage to this method of production of ozone is that glassy carbon electrodes are costly.
U.S. Pat. No. 4,416,747 (Nov. 22, 1983) Menth, Anton, et al. Process for the Synthetic Production of Ozone by Electrolysis and Use Thereof, outlines a process for production of ozone by electrolysis in which the produced ozone is used in water treatment. The anode and cathode are made of stainless steel, and between the anode and cathode is a solid electrolyte made of a plastic polymer based on perfluorinated sulphonic acids. The solid electrolyte serves as a thin ion-exchange membrane which is coated on the cathode side with a layer of a mixture of 85% by weight carbon powder and 15% by weight platinum powder. The anode side of the membrane is coated with PbO2 powder. A solution of oxygen-saturated water is fed into the cell, and ozone is produced in the solution on the anode side of the solid electrolyte ion-exchange membrane while water is formed on the cathode side. The H+ which is produced on the anode side by the decomposition of water to form oxygen and ozone migrates through the ion-exchange membrane and reacts with oxygen in the water on the cathode side to form water. The evolution of hydrogen at the cathode is thereby suppressed.
U.S. Pat. No. 4,541,989 (Sep. 17, 1985) Foller, Peter C. Process and Device for the Generation of Ozone via the Anodic Oxidation of Water describes using an electrolytic cell in which an air cathode reduces the oxygen in air to water, and an inert anode decomposes the water to ozone at claimed levels of ten pounds per day by electrolysis using DC current.
U.S. Pat. No. 5,154,895 (Oct. 13, 1992) Moon, Jae-Duk. Ozone Generator in Liquids suggests an ozone generator consisting of one or more pairs of strip electrodes made of an oxidation resistant metal such as Pt, PbO2, or SnO2 mounted on a substrate inside an ozonizing chamber with outer terminals extending outside the ozonizing chamber. The chamber has an inlet for a liquid such as water or solutions of H2SO4, HClO4, HBF4, or H3PO4. An electric current is supplied to the electrodes through the terminals outside the chamber, and water molecules are dissociated at the electrodes producing ozone gas in the liquid without use of the conventional blower to supply carrier air to the ozone generator.
U.S. Pat. No. 5,203,972 (Apr. 20, 1993) Shimamune, Takayuki, et al. Method for Electrolytic Ozone Generation and Apparatus Therefor describes an electrolytic cell wherein the electrolyte separating the anode and cathode is a solid electrolyte, preferably a perfluorocarbon sulfonic acid-based ion-exchange membrane. The anode is made by covering a titanium substrate first with a coat of platinum, gold, or like metal, and then with an electrodeposited layer of lead dioxide. When an electric current is passed through the cell ozone is formed at the anode in an ozone resistant chamber made of Teflon® or titanium.
U.S. Pat. No. 5,332,563 (Jul. 26, 1994) Chang, Shih-Ger. Yellow Phosphorus Process to Convert Toxic Chemicals to Non-Toxic Products outlines a process which involves passing air or oxygen over aqueous emulsions of yellow phosphorus, P4, which results in the formation of P4O10 or P2O5, and an abundance of reactive species such as atomic oxygen and ozone. This process is a development from the disclosure of U.S. Pat. No. 5,106,601 (Apr. 21, 1992) which outlines a method for removing acid-forming gases such as NO and NO2 from exhaust gases. Ozone is produced in the process. In both cases production of P4O10 or P2O5 results when the phosphorus combines with oxygen molecules and a large amount of atomic oxygen is detected in area of the reaction. The atomic oxygen may combine with oxygen molecules to form ozone.
U.S. Pat. No. 5,460,705 (Oct. 24, 1995) Murphy, Oliver J., et al. Method and Apparatus for Electrochemical Production of Ozone describes an electrochemical method and apparatus for production of ozone which uses an anode made up of a substrate made from porous titanium, titanium sub-oxides, platinum, tungsten, tantalum, hafnium, niobium, or similar material, and a catalyst coating selected from lead dioxide, platinum-tungsten alloys, glassy carbon or platinum. The cathode is a gas diffusion cathode consisting of a polytetrafluoroethylene-bonded, semi-hydrophobic catalyst layer supported by a hydrophobic gas diffusion layer. The catalyst layer consists of a proton exchange polymer, polytetrafluoroethylene polymer, and a metal such as platinum, palladium, gold, iridium, or nickel. The anode and cathode are separated by an ion-conducting electrolyte which is a proton exchange membrane (PEM) with one side bonded to the catalyst layer of the gas diffusion cathode and a second side touching the anode. An electric current is passed through the anode and the gas diffusion cathode, and ozone is formed at the anode.
An international patent application, WO 2004/072329 (26 Aug. 2004) Cheng, S., et al, Device for and Method of Generating Ozone describes an electrode made from a substrate selected from titanium, gold-coated titanium, and other inert conducting materials, with a coating of tin dioxide modified by antimony. The coating may also include nickel. The coating may comprise particles of from 3 nm to 5 nm in size and in a ratio of Sn:Sb in the range of from about 6:1 to 10:1. Multiple coatings may be applied to the substrate, e.g. by dip-coating and heat treatment steps. The electrode is suitable for direct generation of ozone in water or through water into a gaseous state.
The use of the electrode is described in a cell containing an electrolyte which may comprise SnCl4.5H2O and SbCl3 in an ethanol-HCl mixture, or which may simply utilize pure water without any dissolved ions. An optional alternative system comprises a solid polymer electrolyte, such as Nafion®.
Whilst this system represents a significant improvement over previous proposals, there remains room for improvement in certain aspects. In particular, the lifetime of the catalyst in prototype systems developed following this patent has been found to be limited to a few days to weeks at best and the innovations presented in this patent result in even higher efficiencies.
Ozone (O3) is a very strong oxidising agent which has many uses, including those shown in Table 1 below.
TABLE 1Some applications for generated OzoneWater TreatmentBottled water andTaste and odourbeveragesimprovementMains drinkingPools and spaswaterBy-productWater reuse andreduction forrecyclingchemicalContaminantdisinfectionreductionWaste TreatmentEffluent andSoil remediationwastewaterand treatmenttreatmentShip ballast waterGround waterAir conditioningremediationrecirculating waterSuspended solidsPower stationreductioncooling waterActivated sludgesFood TreatmentDisinfectionPreservationSterilisationStorageDeodorisationGrain treatmentBleachingPaperWaxesSynthetic fibresFlourTeflonDecontamination andHospitalsInfectious agentdisinfectionremoval
Chlorine-based products are used for many of these purposes, but they can, in some circumstances, lead to the production of carcinogens such as trihalomethanes and chloramines. Chlorine can be unpleasant to users (e.g. in swimming pools) and can directly contaminate the environment. For example, chlorine-based products are banned from use as a bleaching agent in pulp and paper mills in a number of countries—its use in this field has fallen from 7% to 1% of total chlorine usage in the US. Chlorine plays a major role in the above markets, which consumed around 20% of total chlorine supply in the US in 2002 (12.5M tons at $230 per ton=$2.9 bn).
Ozone is a safe alternative to treatment by chlorine or chlorine-based products. It performs the same functions without the undesirable side effects; it is not harmful to the environment since it rapidly decomposes into oxygen, O2.
The electrochemical generation of ozone depends critically on the proper choice of electrode material and catalyst. Using conventional anodes, such as Pt, and imposing a sufficiently positive potential on the electrode immersed in aqueous solution will, under normal circumstances, result in the generation of oxygen, according to:2H2O→O2+4H++4e−, E°=1.23 V.  (1)
Ozone generation relies on suppressing this reaction (e.g. by producing an anode with a “high oxygen overvoltage”) such that ozone can then be produced in preference thus:3H2O→O3+6H++6e−, E°=1.51 V, and:  (2)H2O+O2→O3+2H++2e−, E°=2.07 V.  (3)
An advantage of this strategy is that any loss in current efficiency for ozone generation will mostly lead to the production of harmless oxygen which, in many cases, can actually be useful.
The water oxidation reaction, equation (1), can be suppressed by careful catalyst design and/or through the choice of experimental conditions, both of which directly influence the intermediate species which determine which of steps (1)-(3) above can take place.
A number of anodes have been investigated with respect to furthering research into realising a commercially useful method for electrochemical ozone generation:
Pt, α-PbO2, β-PbO2, Pd, Au, dimensionally stable RuO2 anodes (RuO2DSA), doped diamond, and glassy carbon (GC) in various electrolytes and under a range of experimental conditions. Au, RuO2DSA and GC anodes all yield current efficiencies of <1%.
The state-of-the-art with regard to development of electrochemical technologies is represented by the work of Putnam et al [G. L. Putnam et al, J. Electrochem. Soc., 93 (1948) 211]; Foller and Tobias [P. C. Foller, C. W. Tobias, J. Electrochem. Soc., 129 (1982) 506]; Cheng et al, [WO 2004/072329 A1 (Aug. 26, 2004)] and Murphy and Hitchens [U.S. Pat. No. 5,460,705 (Oct. 24, 1995)].
An object of the present invention is to provide improvements in the production of ozone and in particular to develop an electrochemical process suitable for commercialisation.